diff --git a/SVSim.UnitTests/Story/StoryServiceTests.cs b/SVSim.UnitTests/Story/StoryServiceTests.cs
index 7e1e20a..1599ad5 100644
--- a/SVSim.UnitTests/Story/StoryServiceTests.cs
+++ b/SVSim.UnitTests/Story/StoryServiceTests.cs
@@ -16,6 +16,11 @@ using SVSim.UnitTests.Infrastructure;
 namespace SVSim.UnitTests.Story;
 
 [TestFixture]
+// One instance per test case so parallel tests don't race on the SetUp-initialised
+// _master / _viewer / _service fields. NUnit's default SingleInstance shares the
+// fixture instance across all tests in the class; under ParallelScope.All, concurrent
+// SetUps wipe each other's Mock setups and we see NullReferenceExceptions in service code.
+[FixtureLifeCycle(LifeCycle.InstancePerTestCase)]
 public class StoryServiceTests
 {
     private Mock<IStoryMasterRepository> _master = null!;
@@ -829,12 +834,15 @@ internal static class StoryServiceTestHelpers
     /// <summary>
     /// Returns a minimal <see cref="SVSimDbContext"/> backed by the EF InMemory provider.
     /// Safe for non-reward tests that never actually query the DB.
-    /// Each call should use a unique <paramref name="dbName"/> to prevent test bleed-through.
+    /// The supplied <paramref name="dbName"/> is suffixed with a fresh Guid so concurrent
+    /// callers never share a database — EF InMemory keys by name, and the previous callers
+    /// all passed the literal string "SetUp" via <c>nameof(SetUp)</c>, which collapsed every
+    /// test in the fixture onto the same store and broke under parallel execution.
     /// </summary>
     public static SVSimDbContext NewInMemoryDb(string dbName)
     {
         var options = new Microsoft.EntityFrameworkCore.DbContextOptionsBuilder<SVSimDbContext>()
-            .UseInMemoryDatabase(dbName)
+            .UseInMemoryDatabase($"{dbName}-{Guid.NewGuid():N}")
             .Options;
         return new SVSimDbContext(
             Microsoft.Extensions.Logging.Abstractions.NullLogger<SVSimDbContext>.Instance,